1. Field of the Invention
The present invention relates generally to wireless communications devices. More specifically, the present invention relates to adaptive filtering techniques for use in mobile communications environments.
2. Related Art
Mobile communications devices, such as wireless phones, are often employed in vehicles or other mobile platforms that travel at relatively high velocities. For example, mobile communications devices are often used in automobiles, trains, and airplanes that travel at speeds as fast as several hundred kilometers per hour.
In wireless communications environments, Doppler frequency shifts occur in signals transmitted between a transmitter and a receiver when the transmitter and the receiver have a relative velocity with respect to each other. This relative velocity causes signals, from the perspective of the receiver, to have frequencies that are shifted from the frequencies transmitted by the transmitter. When the relative velocity is an approaching velocity, these frequency shifts are upward. However, when the relative velocity is a departing velocity, these frequency shifts are downward.
Many components in wireless communications devices, such as filters, are configured to receive signals within a particular frequency range. Unfortunately, Doppler frequency shifts can place such signals outside of these configured frequency ranges. This condition can lead to degraded performance.
For example, in wireless communications systems that operate according to code division multiple access (CDMA) standards, such as IS-95A and IS-95B (referred to herein as IS-95 systems), signals known as pilot signals are transmitted from base stations to mobile devices. These pilot signals provide timing and phase references that enable these mobile devices to receive and demodulate traffic signals carrying information such as data and voice that are also transmitted by the base station.
Since these pilot signals are used as timing and phase references, their signal-to-noise ratio (SNR) directly affects a device's ability to receive and demodulate traffic signals.
Wireless communications devices typically filter pilot signals with a pilot filter having a fixed bandwidth. Because mobile communications environments often involve Doppler frequency shifts, portions of a received pilot signal's spectrum can fall outside of the pilot filter's fixed bandwidth. This causes distortions that reduce the pilot signal's SNR.
On the other hand, if the pilot filter's fixed bandwidth is sufficiently large to accommodate such Doppler frequency shifts, out of band noise can be introduced to the pilot signal reception processor. Unfortunately, this introduction of noise also reduces the pilot signal's SNR.
Accordingly, what is needed is a technique that enables pilot filtering to be performed in a manner that accommodates Doppler frequency shifts, while not causing a SNR reduction in pilot signals.